1. Field of the Invention
This invention relates to a magnetic head for use with magnetic recording media including floppy disks, magnetic tape, and hard disks, and more particularly to a write or read/write magnetic head
2. Prior Art
In writing and reading information in magnetic recording medium, it is preferred to use a magnetic head having a wide gap to provide an intense magnetic flux during writing But, it is preferred to use a magnetic head having a narrow gap to increase resolution during reproduction. A magnetic read/write head has a fixed gap length which must be a compromise between these two considerations.
One approach is the magnetic heads disclosed in U.S. Pat. No. 4,646,184 and corresponding Japanese Patent Application Kokai No. 60-87411, which are now known as dual gap length (DGL) type and enhanced dual gap length (EDG) type heads.
Referring to FIG. 1, there is illustrated a typical configuration of a DGL type magnetic head. First and second cores 1 and 2 are joined into a one-piece member through a nonmagnetic spacer 4 Disposed between the first core 1 and the nonmagnetic spacer 4 is a first thin film 3 having a lower saturation magnetic flux density than the cores. Two first thin films may be disposed on opposite sides of the spacer 4
FIG. 2 illustrates a typical configuration of an EDG type magnetic head The EDG head has generally the same structure as the DGL head of FIG. 1 In addition to the first thin film core and the nonmagnetic spacer 4. Two first thin films and two second thin films may be disposed on opposite sides of the spacer 4.
These DGL and EDG magnetic heads operate well both during writing and reading. During recording, the first or low saturation flux density thin film 3 is magnetically saturated so that it serves as a gap together with the nonmagnetic spacer 4, thus ensuring writing with a wider gap. During reproduction, the leakage magnetic flux that the magnetic recording medium develops during reproduction is too minute to magnetically saturate the first or low saturation flux density thin film 3. Thus the thin film 3 serves as a part of the first core 1 to ensure reproduction with a narrower gap, resulting in a high resolution. Particularly the EDG magnetic head ensures effective recording of a magnetic recording medium having a high coercive force because the second or high saturation flux density thin film 5 enables the application of a strong magnetic flux to the magnetic recording medium during writing.
In connection with these DGL and EDG magnetic heads, U.S. Pat. No. 4,646,184 proposes to form the low saturation flux density thin film 3 from a soft magnetic oxide material such as garnet. The second or high saturation flux density thin film 5 is disclosed as being formed from a crystalline soft magnetic material such as Permalloy and Sendust or an amorphous soft magnetic material Ferrite is used as the core.
To meet the above-mentioned requirements, the low saturation flux density thin film 3 must have a lower saturation magnetic flux density, a higher initial magnetic permeability, and a lower coercive force than ferrite commonly used as the core.
However, soft magnetic oxide materials have a lower limit of saturation magnetic flux density because of their crystal structure. To reduce the saturation magnetic flux density below the lower limit, particularly to 3,000 Gauss (G) or lower, a nonmagnetic material must be added to the soft magnetic oxide material. However, the nonmagnetic material added is present as islands in the crystal structure, leading to a drastic reduction of initial magnetic permeability. Although magnetic head materials prefer low coercive force, formulating soft magnetic oxide material to a low coercivity composition fails to provide the desired saturation magnetic flux density.
There are available no soft magnetic oxide materials which can meet all the requirements of low saturation magnetic flux density, high initial magnetic permeability, and low coercive force Currently available soft magnetic oxide materials are more or less unsatisfactory in read/write performance. More particularly, the magnetic heads of soft magnetic oxide material have a low reading output, and tend to be magnetized as a result of repeated cooperation with magnetic recording medium, resulting in a further reduction in reading output as well as a variation in electromagnetic properties. Since magnetization occurs if a soft magnetic oxide material assumes its inherent crystal structure, conditions for crystal growth must be carefully controlled Subsequent heat treatment is often necessary. These add to the difficulty of manufacture.
The soft magnetic oxide material shown in the example of said patent has a saturation magnetic flux density of 1,000 G. The soft magnetic oxide material having such an order of saturation magnetic flux density exhibits no effective magnetic permeability at room temperature because its Curie point is as low as about 40.degree. C. Thus heads formed therefrom exhibit unstable magnetic properties and a low resolution. Also a problem arises upon overwrite recording.
The same discussion applies to ferrite.
Various crystalline metal materials are known as the soft magnetic material having a low saturation magnetic flux density. However, none of the thin films formed from these crystalline metal materials can meet all the requirements of low saturation magnetic flux density, high initial magnetic permeability, and low coercive force as the above-mentioned oxide thin films do not. Heads formed therefrom exhibit inconsistent electromagnetic properties and insufficient read/write properties.
The high saturation magnetic flux density thin film is formed from crystalline metal material such as Sendust according to the disclosure of said patent. A thin film which is formed on the core from such crystalline metal tends to be stressed during film formation since the crystal structure of the crystalline metal is different from that of the coreforming ferrite. Reaction can sometimes occur between the crystalline metal and the ferrite during subsequent heat treatment or glass fusion bonding. For these reasons, a pseudo-gap can form at the interface between the crystalline metal and the ferrite, that is, between the high saturation magnetic flux density thin film and the core, adversely affecting read/write properties, particularly reading properties.
A DGL or EDG magnetic head as shown in FIGS. 1 and 2 is generally formed by bonding the first and second cores 1 and 2 through the nonmagnetic spacer 4 and the thin films 3 and 5 into a one-piece member Bonding is preferably carried out by fusion welding with glass from the point of view of increased durability and productivity, particularly when a high mechanical strength is required for the head.
Where the low saturation magnetic flux density thin film 3 is formed from an amorphous material, it is very difficult to bond such a thin film to the core by fusion welding with glass. Where the low saturation magnetic flux density thin film 3 is formed from a crystalline material, fusion welding may employ a conventional glass having a relatively high working temperature Tw at which its viscosity reaches 10.sup.4 poise. This is because heating at a relatively high temperature does not substantially affect the magnetic properties of the thin film.
In contrast, amorphous material crystallizes above a certain temperature. Thus welding with a conventional glass having a relatively high working temperature Tw causes the amorphous material to crystallize. The resulting thin film exhibits an increased saturation magnetic flux density, an increased coercive force, and a reduced initial permeability, failing to provide satisfactory read/write properties.
Integral bonding of cores into a magnetic head must use a welding glass having a low working temperature Tw at which amorphous material does not crystallize. There is a need for a welding glass composition having such a low working temperature.